To move beyond modern electronics, increasing attention has turned to spintronics, which utilizes both the charge and spin degrees of freedom of electrons. Spintronics has largely focused on ferromagnets due to their strong electrical responses. Recently, however, antiferromagnetic systems have attracted significant interest for their potential to operate at much higher frequencies. Despite this promise, their typically weak electrical signals present major challenges for practical implementation.

Mn₃Sn is an antiferromagnet featuring an inverse triangular spin structure on a Mn kagome lattice. This spin arrangement breaks macroscopic time-reversal symmetry and can be described by an octupole polarization. Mn₃Sn exhibits strong ferromagnet-like responses, such as anomalous Hall effect, despite having negligible net magnetization. Recent progress in fabricating epitaxial W/Mn₃Sn bilayers has enabled electrical manipulation of this spin structure via spin-orbit torque generated by the spin Hall effect in W. This advancement makes the W/Mn₃Sn system a promising candidate for antiferromagnet-based spintronic devices. However, it remains unclear whether the observed spin-torque-induced effects are intrinsic to the W/Mn₃Sn interface or are influenced by ferromagnetic secondary phases.

To address this issue, we conducted x-ray magnetic circular dichroism (XMCD) measurements. While XMCD is conventionally used for probing ferromagnetic materials, we have recently shown that it can also detect the inverse triangular spin structure of Mn₃Sn through the magnetic dipole term [1]. In this study [2], we employed both surface-sensitive total electron yield (TEY) and bulk-sensitive partial fluorescence yield (PFY) modes to investigate the uniformity of the antiferromagnetic spin structure (or octupole polarization) throughout a W/Mn₃Sn/MgO multilayer, from the bottom to the top interface.

The epitaxial Mn₃Sn thin film was grown by molecular beam epitaxy on an MgO($100$) substrate, with the final structure comprising MgO($100$)/W (7 nm)/Mn₃Sn (30 nm)/MgO (3 nm). XMCD measurements were carried out at beamline BL25SU at SPring-8 at room temperature. In TEY mode, the drain current compensating for photoemitted electrons was recorded, while in PFY mode, fluorescence x-rays were detected using a silicon drift detector (SDD), as shown in Fig. 1. The probing depths of TEY and PFY are approximately a few nanometers and $\sim$100 nm, respectively—thus, TEY probes the surface region, while PFY covers the full 30-nm-thick Mn₃Sn layer.

Figure 2 presents the Mn $L$₃-edge XMCD spectra acquired using TEY and PFY modes under an applied magnetic field of 50 mT. Prior to this, a field of 1.9 T was applied to saturate the octupole polarization. Both spectra show a distinct positive peak at 638.7 eV, characteristic of the inverse triangular spin structure. Notably, the TEY and PFY spectra are nearly identical, demonstrating uniform electronic and magnetic structures across the entire Mn₃Sn layer from the bottom W/Mn₃Sn and the top Mn₃Sn/MgO interfaces. Furthermore, the absence of any signatures associated with ferromagnetic secondary phases confirms the structural and magnetic purity at the W/Mn₃Sn interface. These results suggest that the spin-torque induced phenomena recently observed in W/Mn₃Sn bilayers are intrinsic in nature and originate from the direct interplay between spin currents and the octupole polarization.

References

• [1] S. Sakamoto, T. Higo, M. Shiga, K. Amemiya, S. Nakatsuji, and S. Miwa, Phys. Rev. B 104, 134431 (2021).
• [2] S. Sakamoto, T. Higo, Y. Kotani, H. Kosaki, T. Nakamura, S. Nakatsuji, and S. Miwa, Phys. Rev. B 110, L060412 (2024).

Authors

• S. Sakamoto, T. Higo^a, Y. Kotani^b, H. Kosaki, T. Nakamura^b,c, S. Nakatsuji^a,d, and  S. Miwa
• ^aThe University of Tokyo
• ^bJapan Synchrotron Radiation Research Institute
• ^cTohoku University
• ^dJohns Hopkins University